Liquefied natural gas, hereafter “LNG”, is natural gas that has been converted at least temporarily to liquid phase for ease of storage or transport. LNG takes up about 1/600th the volume of natural gas in the vapor or gaseous phase. The reduction in volume makes it much more cost efficient to transport over long distances where pipelines may or may not exist. In certain cases where moving natural gas by pipeline is not possible or economical, LNG can be transported by specially designed cryogenic sea vessels known as “LNG carriers” or “cryogenic road tankers”.
The conventional regasification process for onshore and offshore plants incorporates two major elements:    1. High-pressure send-out pumps to bring the pressure of the LNG up from relatively low storage pressure through the vaporizer to relatively high pipe line pressure; and    2. The vaporizer/expander to transform the LNG into gaseous natural gas.
The LNG regasification process consists of the steps of unloading LNG vessels at the receiving terminal and storing LNG in insulated tanks at atmospheric pressure at a temperature in the range of 111 Kelvin [K], which is around minus 170 Celsius [C]. During the regasification process, LNG is pumped to a high pressure by a cryogenic, high-pressure LNG pump or similar equipment while it is still in the liquid state. The LNG is then heated until it vaporizes into its gaseous state. In common commercial practice, the heat source used in regasification of LNG is provided by local sea water. The naturally stored “heat” in sea water is a heat source for heating and vaporizing LNG.
Cryogenic high-pressure LNG pumps are used for pressurizing the fluid up to the high pipe line pressure while it is still in the liquid state. Typical dimensions for these types of pumps are 4 meters in height and 1 meter in diameter, with as many as 12 or more centrifugal pump impeller stages, each of up to 300 mm or more diameter.
FIG. 1 (prior art) shows a particular design of an existing high-pressure, centrifugal LNG pump. Its design features the single piece, rotating axial shaft with integrally mounted multi-stage pump hydraulics and electrical induction motor. The thrust balancing mechanism is incorporated into the pump to eliminate high axial thrust forces on the bearings. The electrical induction motor is submerged in and cooled by LNG, and the ball bearings are lubricated and cooled by LNG.
General, high-pressure pump design criteria is summarized as follows:
Pump General Design CriteriaLiquidLNGModel6ECC-1212Pump Design Pressure[bara]133.4Lowest Design Temperature[° C.]−168Operating Temperature[° C.]−147Rated Flow[m3/hr]287Rated Differential Head[m]2396Rated Density[kg/m3]417.417Maximum Design Density[kg/m3]451.00
Typical LNG regasification plants require large heat sinks that necessitate large heat sources. Temperature differentials between heat sources, e.g., seawater, and heat sinks, e.g., LNG, are in the range of 170° Celsius, thus providing feasible preconditions for an efficient recovery of power. There have been past attempts to recover some of the input energy used during the LNG regasification process. One common limitation is that the energy recovered using a common one-phase [liquid] turbine and generator combination, from just the vapor or gaseous state of LNG, is very ineffective. However, in a two-phase [liquid, gas] process, the combination of high pressure and mechanical turbulence created and the presence of liquid droplets of LNG is highly corrosive or abrasive, and will damage the equipment used in most current regasification plants.
The Rankine power cycle is a cycle that converts heat into work. Heat is supplied externally to a closed loop of working fluid, such as water. The working fluid is heated, vaporized, used to drive a steam turbine to generate electrical power, re-condensed to liquid by cooling, and the cycle is repeated. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants.
The Rankine power cycle describes a model of steam operated heat engine most commonly found in power generation plants. Common heat sources for power plants using the Rankine power cycle are the combustion of coal, natural gas and oil, and nuclear fission.
There is nothing in the prior art that teaches a system which converts heat from a heat source to work incorporated into an LNG regasification process. The need exists to incorporate a reduction generator, where the work is further converted into electrical energy, to recovers some of the energy input to the LNG during the regasification process. There are currently no power plants in operation in which the working fluid is LNG and the heat source is mainly sea water, wherein the entire process operates at a much lower temperature than that utilized in conventional power generation plants.